1. Field of Invention
The present invention relates to the mechanical driving system of an optical scanner. More particularly, the present invention relates to a gear-shifting device for changing the scanning speed of an optical scanner.
2. Description of Related Art
An optical scanner is one of the most frequently used computer peripheral devices. By converting an optical image into digital data, the scanner is able to store the image in a digital format that can be readily modified on demand. Data is extracted from an object image by projecting a beam of light from a light source to the object image. Upon reflection from the object image, the light passes through a set of optical lenses. Ultimately, the light is intercepted by a charge-coupled device (CCD) and converted into digital signals. In general, the resolution of an optical scanner depends on the level of integration and sensitivity of the CCD. However, the resolution of an optical scanner is also closely related to the scanning mechanism of the scanner. A platform type scanner typically includes a charge-coupled device (CCD) and a drive system. The scanner is usually driven by a stepping motor. The CCD can be attached to the drive system so that scanning is carried out directly. Alternatively, the CCD can be fixed in position while a group of optical lenses is attached to the drive system so that a scanning light beam is transmitted to the fixed CCD after passing through the group of optical lenses. In general, the CCD is able to read a scanning line (also known as a horizontal line) of image data in each scanning interval with each scanning line consisting of a plurality of pixels. Resolution, also known as horizontal resolution or optical resolution, of each scanning line depends on the density of sensors in the CCD. Scanning is carried out vertically through the CCD directly attached to the drive system or the group of optical lenses attached to the drive system. Vertical resolution, also known as motor resolution, is related to the stepping angle of the stepper motor and precision of the drive system. Through vertical scanning, a plurality of lines of image data are formed. The data from all these image lines can be combined to form a complete image.
FIG. 1 is a schematic diagram showing the drive system of a conventional optical scanner. The drive system of an optical scanner generally includes a drive motor 100 such as a stepper motor. Power transmission components 102 such as gears, screw rods or a leather belt are also employed to drive an output unit 104 such as a CCD or a group of optical lenses. Hence, the optical scanner is able to capture data line by line. For practical reasons, the optical scanner must be able to provide different scanning resolutions. However, the power transmission components 102 of most conventional optical scanners have only one set of gears and hence provide only one gear ratio. Hence, it is impossible to select between a rapid scan and a high-resolution scan.
Accordingly, one object of the present invention is to provide a speed-changing gear set for an optical scanner so that the driving system can have two or more gear ratios to perform high speed scanning or high resolution scanning.
To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention provides a speed-changing gear set installed within a driving system of an optical scanner. The gear set includes a control element, a cam, an axle, a first set of gears and a second set of gears. The cam is connected to the control element. The cam is able to rotate when driven by the control element. The first set of gears is mounted on the cam and coupled to the output unit of the driving system. The axle is attached to the cam so that the axle is able to shift position when the cam is rotated. The second set of gears is mounted on the axle. When the cam is driven to a first position, the first set of gears engages directly with a gear on the driving motor, thereby driving the output unit. On the other hand, when the cam is driven to a second position, the axle shifts to another position such that the second set of gears engages with the gear on the driving motor and the first set of gears. Hence, the output unit is driven by the driving motor via the first and second set of gears.
According to one embodiment of this invention, the control element is a control motor. The cam has a gear section and a curve section. The gear section engages with a gear on the control motor. The first set of gears includes a first gear, a second gear and a third gear. The first gear and the cam are on the same axial. The first gear engages with the gear at the output unit. The second gear is mounted on an axle that is inserted into the cam and hence follows the movement of the cam. The second gear and the third gear are coaxial. The second and the third gear are fixed next to each other so that they rotate in step together. The third gear engages with the first gear. The axle cuts across the curve surface section of the cam. In addition, an elastic device pushes the axle against the curve surface in a direction toward the center of the cam so that the axle can slide along the curve contour of the cam when the cam rotates. The second set of gears includes a fourth gear and a fifth gear both mounted on the same axle. The fourth gear and the fifth gear are joined together so that they move in step together.
When the control motor drives the cam to a designated first position, the axle will be on the first position of the cam surface, while the second gear will engage with the gear on the driving motor. Force is transmitted from the driving motor through the second gear, the third gear and the first gear to the output unit.
When the control motor drives the cam to a designated second position, the axle will be on the second position of the cam surface. Hence, the fifth gear meshes with the gear on the driving motor, while the fourth gear meshes with the second gear. Force is transmitted from the driving motor through the fifth gear, the fourth gear, the second gear and the first gear to the output unit. In this embodiment, the range of cam movements is limited by two stopping blocks.
The speed changing gear set of this invention can provide at least two gear ratios. Hence, a high resolution but slow speed scanning can be executed to obtain an accurate image. On the other hand, by shifting the cam to a higher gear ratio, a lower resolution is obtained, but scanning is completed in a shorter period. Therefore, performance capacity of the optical scanner improves.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.